Attenuation Theory
by McStories
Summary: A transporter mishap results in kid!Chekov being let loose on the Enterprise. McCoy reluctantly appoints himself babysitter as Spock and Scotty struggle to find a way to reverse the change.


Title: Attenuation Theory  
Author: Gloria  
Pairing: Unsure as of now. Possibly McCoy/Chekov, but it would be a while. It's McCoy and Chekov centric, anyway.  
Rating: R, to be safe  
Summary: A transporter mishap results in Kid!Chekov being let loose on the Enterprise. McCoy appoints himself babysitter as Spock and Scotty struggle to find a way to reverse the change.  
Notes: Inspired by the Babies and Hyposprays request on the Star Trek Kink Meme. (Request was for a transporter accident to result in BB!Chekov running around the ship.)

* * *

When Pavel Chekov ran through the corridors every morning in his uniform undershirt and civvie track pants, he was like the crew's own version of a sunrise. He would smile at every passing face, neatly dodge the dragging feet of the crew coming off the night shift, hum tunelessly to himself, glowing and sweaty.

The shining example of good health and McCoy used him as an example for any particularly lazy, out of shape idiots who claimed there was no way to stay active on a starship.

When Chekov ran in the mornings as if the narrow rounded corridors were a training field, it made people smile.

When Chekov ran down the corridors any other time, people got the fuck out of his way.

They learned that from the very start, when the kid had pounded through the halls and barreled around turns, making the run from the bridge to the transporter room – almost half the length of the ship – just in time to save Kirk and Sulu from a messy death.

Two months in to their official five-year assignment, Chekov had flung himself across the even greater space from his own quarters to the bridge just in time to diagnose and fix the malfunction in the sensor array that had Kirk five seconds from destroying a simple cargo ship that the computer said had all weapons armed and ready to fire.

McCoy had only actually seen the kid in mid-tear a few weeks after that, when damage from enemy fire was threatening the entire crew. Blah blah engineering jargon blah blah instant death - McCoy didn't like to pay attention to the details in most of those situations. He'd been on his way from the bridge to sickbay to prepare for some serious radiation sickness when this slender gold and black blur whizzed past him, and he heard a string of Russian-tinged words hanging in the kid's wake like footsteps marking his trail.

When Chekov ran through the corridors in mid-shift, people flattened against walls like magnets sucked to metal. Nobody wanted to be the asshole that knocked the kid over, slowed down whatever brilliance he was on his way to do, and ended up getting everyone killed.

Funny thing was, the kid was so damned fast that it didn't leave a person much time to worry about what exactly had him skidding through the halls. Plans seemed to fall into his brain fully formed, and when he showed up in the transporter room to save Jim and Sulu, or on the bridge to save a cargo ship from destruction, he raced right to where he needed to be, let his hands fly over the controls for a minute or two, and then. Boom. Problem solved, chaos over.

That was how Chekov operated. That was how his genius worked, in spurts so instant and thorough that McCoy had heard even Spock expressing interest.

That was also why, when McCoy found himself on the receiving end of one of Chekov's running saves, he only had the slightest moment of warning that anything was even wrong.

He was set to transport down to some new planet – blah blah alien culture blah blah diplomacy – because some idiot – probably Jim - on the landing party that had taken the shuttlecraft down earlier was already bleeding out and needed saving.

He was on the transporter pad, medkit clenched in his technophobic fist as he waited for the dreaded moment of transport.

The doors slid open. Gold-and-black-blur flew in and bumped into Scotty hard enough to knock a padd out of his hand. Even as the tingle of the transporter started filling McCoy's vision the blur launched itself at the pad and plowed into him.

McCoy was a pretty solid guy, but he was knocked off the pad and onto his ass, and the medkit went flying into the wall. He didn't see what the hell happened, but by the time he had been helped off the ground the pad was empty, deserted, and Scotty and a couple of his engineering lackeys were staring at the control panel and talking in urgent, clipped voices.

"What the hell was that?" he asked with his usual patience, marching up to the controls and scowling around him for the little Russian battering ram.

The look on Scotty's face made him shut up fast, though, which was how he heard Jim's tinny voice coming through the communicator on the panel.

_"Scotty, say again. You've got him, right?" _

Scott's fingers were fast and smooth over the controls, but nothing he was doing had any noticeable affect on anything, and after another minute of ignoring McCoy's stare and Jim's nagging voice his hands slowed, and his shoulders slumped.

He spoke to Jim, his eyes on McCoy. "Captain...the transporter is still showing active. That's why ye cannae see him on your end."  
_  
"Still active? What's that mean? Still in process?"_

"Aye, sir." Scotty looked past McCoy, his expression stark.

Reflexively, McCoy followed his gaze back to the empty transporter pad.

"Aye," Scotty said again behind him. "According to the sensors, he's stuck. Lodged up against the planet's atmosphere." It was strange hearing grimness in Scotty's lilting tenor, but it was getting stronger by the moment. "I cannae retrieve him until the computer completes the transport, but it's well and truly stuck."

A pause, and then a low, level voice came through the communicator. _"Have you tried canceling the transport, Mr. Scott?"_

"Begging your pardon, Commander, but if I wanted to be a murderer I'd find a more direct way to go about it."

_"I think it highly unlikely that--"_

"If I cancel the transport the computer won't just bloody well reverse itself. It'll cancel. The lad would just...dissipate."

McCoy stilled as he realized what they were saying. He stared at the transporter pad he'd been standing on minutes ago, horror making him heavy.

_"We're coming back, Scott. You and Spock will figure this out."_

"Jim..." It was a croak, strange, and McCoy knew it wouldn't carry over to the communicator.

But even if he didn't hear, Jim responded. _"We're bringing the shuttle. Don't touch that fucking control again until we're up there._"

"Aye, sir."

The comm went silent, and McCoy stood there thinking about that weird jogging Russian kid and how fucking fast he was when he knew there was a problem.

"Sir, you dropped this." One of the engineering ensigns broke the silence.

McCoy turned away from the transporter, stiff and unsure exactly what the hell he was supposed to be doing, or feeling. He watched the ensign pick up that padd that had clattered to the ground when Chekov ran into Scotty.

Scotty stared at the panel, waving him off. "'s not mine, Jumai. You think if we get a good sensor reading of the atmosphere of that planet we could compensate for any disturbance that might be..." Scotty trailed off, unsatisfied by wherever his mind took that theory.

McCoy spoke hesitantly, fairly sure he didn't actually want an answer. "Where exactly is Chekov?"

Scott rubbed at his neck, staring at the panel like he expected it to give up an answer spontaneously any minute. "Nowhere. That's the problem, doctor."

"You're telling me that kid is stuck in mid-transport? Just a bunch of atoms floating around waiting to be put back together?" And you're telling me that was almost me? McCoy wasn't selfish enough to say that last part out loud, of course.

"Sir?"

Scott looked up and flashed a dark, humorless look at McCoy. "That would be the optimistic hope, yes. I've never had a transport just...pause like this. I've got no idea how long the computer will even hold on to...those atoms."

"Jesus Christ." McCoy almost turned back to the empty transporter pad.

"Sir?" The ensign, Jumai, spoke again. He was looking at the padd he'd picked up off the ground, his brow furrowed. "This is..."

"For shite's sake, lad, it's not mine. It must have been..." Scotty jerked as if shocked, leaning over an grabbing the padd from Jumai. "Chekov must've had it with him when he..." He trailed off, staring at the display that had confused Jumai.

McCoy moved around the control, because he needed to do something besides stand there and breathe hard and think about the fact that he should have been floating around in atomic limbo right then, and not Chekov.

He peered over Scott's shoulder at the padd, and saw a string of mathematical formulas and a few notes scribbled shakily around them. Must've used a stylus, but the kid was usually neater than...

Jesus. He probably wrote this in mid-run.

Scotty whistled suddenly, paging down through the display. "Fecking hell." He leaned in and slammed the button on the communication panel. "Scott to the bridge."

A familiar mild voice answered. "Sulu here, Scotty. What's going on down there?"

"Never mind that. Whoever's sitting nearest the science station, I need an exact map of that damned planet's atmosphere. Chemical composition and mass, detailed as you can get. The faster you get that down here the more likely we'll be able to recover your little Russian friend with all his bits intact."

* * *

By the time Kirk and Spock strode through the doors into the transporter room, McCoy was forgotten against a wall listening to Scotty and his cabal of nerds dissecting that padd and comparing to readouts the bridge was sending down at a near-continuous pace.

"Spock!" Scotty ignored Jim – he was one of the few who'd get away with that – and waved the Vulcan over. "How much do you know about attenuation theory?"

Spock didn't miss a beat, going to the panel and taking the padd Scott held out. "In the matter of optics, or telecommunications?"

"In the matter of absorption coefficients. Look here." Scott pointed at some random line on the padd.

Jim hung back, scowling at being ignored but smart enough not to interfere with the scientists while they were sciencing. He spotted McCoy and headed over, thrumming with tension that told McCoy he wouldn't be content to be on the sidelines for long.

"Got any idea what's going on here?" Jim asked when he was close enough.

McCoy shrugged, feeling bleak. "Scott's been throwing around fancy words for a while now. Hell if I can understand half of it."

But he did. He understood enough, though like most things on this ship the actual mechanics were way outside his field of expertise.

"I think," he added slowly, watching Spock's eyebrows get higher with every line of that padd he read, "the general gist is that Chekov got himself absorbed into the atmosphere of that planet and we can't get him back out."

Jim's eyes flashed over to the murmuring engineers and Spock. "Okay, report." His voice was sharp, no nonsense.

Scotty straightened unconsciously, still half-facing Spock even as he responded. "Looks like there's a reason the emissaries from this bloody planet suggested you take the shuttle down. Do you know what the Karman Line is, sir?"

To McCoy's surprise, Jim did. "Sure, it's the boundary between earth's atmosphere and outer space."

"Not exactly, captain." Spock didn't lift his eyes from the padd. "It's an arbitrary measurement that earth scientists use as a boundary because no such boundary actually exists. On earth there is no one place where atmosphere ends and space begins, there is simply less and less atmosphere the higher--"

"The point, Spock. Now."

Spock looked up at that. "The point is that this planet has one. A boundary. I would guess it was the source of the shuttle's strange turbulence as we both landed and left the planet. It seems Ensign Chekov was studying the planet's atmosphere and was making deductions about the composition and he stumbled across this line for himself."

"Stumbled across?"

"It's not something ye could just see, captain, we're nae talking about visible layers. Not only is it invisible, it would be only nanometers thick. Barely thicker than a few rows of atoms." Scotty flashed a smile that was tight around the corners. "It's similar to absorption bands of carbon dioxide you could find in earth's atmosphere, but a tad more...aggressive."

One reason McCoy liked Jim as a captain was that Jim had very little tolerance for talk: "Get to the point, unless you're seriously trying to tell me that the planet ate Chekov."

Spock sighed. "Captain, engaging in hyperbole is neither an effective way--"

"How do we get him back, Spock?"

Spock fell silent. He looked back at the padd. "It appears Ensign Chekov had a formula completed that should offer a solution. I would prefer more time to study it, but..."

"But what?"

Scotty answered. "Well, sir, if we assume the lad's theory is correct about the chemical compound of this planet's version of a Karman Line, it should be that the organic compounds of Chekov's atoms have been absorbed into this thin layer of atmosphere. But with absorption you can also get refraction, and we have no way of knowing the saturation point of the atmosphere or what other compounds it may absorb before it starts refracting away."

McCoy spoke, his voice hoarse. "You mean if we don't act fast the atmosphere's going to start shaking off this kid's atoms bit by bit?"

Spock lowered the padd finally, brow furrowed in thought. "In a matter of speaking, doctor. Yes."

"Jesus _christ_."

Jim only needed a few seconds to consider the matter. Another thing McCoy admired about him, because if it was McCoy who had to make the call he'd still be asking about every little point he didn't entirely understand. Jim had a keen sense for when an instant decision was necessary, and he had the balls to make those decisions.

McCoy didn't envy him.

Jim frowned at the padd in Spock's hand, though he knew better than to waste all their time looking at it himself. "This formula Chekov came up with...?"

Spock's eyebrow twitched. "There are a great deal of variables I haven't had time to study, but..." He looked down at the padd. "I have worked with Ensign Chekov closely and I know his mind. If speed is required we'd do well to trust his work."

"Fine. Do it." Jim spoke confidently, as always committed to his decision the instant it was made. "Get him back here."

Spock and Scotty huddled at the panel with Jumai and began going back and forth in rapid-fire nerd-speak.

Jim slumped against the wall beside McCoy. "This was supposed to be an easy one, damn it."

"That's usually the first warning sign." McCoy grumbled. In the pause he remembered the reason he'd been about to transport down in the first place. "Who was hurt down there?"

"O'Reilly. Chapel met us in the shuttle bay, he's getting patched up." Jim frowned over at McCoy. "There's a reason Chekov was transporting down instead of you, I take it."

McCoy laughed, hollow and shaky, but went over the entire thing for Jim. Not that there was much to tell. The kid came flying in, pushed him out of the activated beam, and got himself ate by a planet.

"And you don't even need to tell me," he finished up, seeing the knowing look forming in Jim's eyes, "that it wasn't my fault. I didn't ask the kid to push me out of the way. But he sure as hell did it to save my ass, and I've got a right to feel guilty about that."

Jim let it go, tilting his head back against the wall as they waited. "We'll get him back, and you'll save his ass next time he's hurt or sick. That's how we operate, Bones."

"Well, if you ask me we could stand to tweak that operation a little bit."

"Captain?"

They both turned to the panel, to Spock.

"We are ready to begin."

"That was quick." Jim pushed off the wall and moved up closer to the transporter pad. "Alright, get us our genius back."

"Er. Captain."

McCoy saw the pale, hesitant look on Scott's face and swallowed down apprehension. "Jim, get back here."

"What?"

"There's a possibility you're not going to want to be too close to..."

Jim's brow furrowed, but cleared with sudden grimness.

In the Academy there had been a few ugly moments, necessary to drive the importance of some of the lessons they taught into the minds of the students. One of the worst, to McCoy at least, was the report and display images from the inside of a transporter room after some officers attempted to beam out through raised shields.

A white room washed in red, and grim words from the ship's doctor about DNA fragments and measuring the mass of the substances cleaned from the walls and floors, assembled buckets of matter, to make sure there was enough volume to account for three people.

As a lesson about safety it was an effective one: McCoy had despised the mere idea of transportation since that day in class.

Jim had taken the same class. Every cadet did. He paled and stepped back. "Try to get his molecules in the right place, guys."

Scotty muttered something Gaelic-sounding that could have been a prayer or a curse, McCoy couldn't tell.

Spock simply pressed the commands to get the transporter into motion.

McCoy wasn't sure if he wanted to look or not. He didn't need a repeat of that first lesson – that one had been effective enough – but whatever would happen to Chekov should have happened to McCoy, and it would be only right to face it dead on.

The column of light began forming on the transporter pad, molecules and atoms getting stuck together. Maybe the right molecules, maybe the right form.

But. No.

God. Right away it was clear something was wrong. The dark mass of shape was too slight, too short and narrow. Half a man. Half of Chekov.

Nausea rose in McCoy's throat, and he wondered if there was any point in going for his abandoned med kit.

He stood where he was, though, and forced himself to watch and remember whatever he saw, to hold on to the last moments of Pavel Chekov.

When the glow of the transporter faded, though, it would have been hard to say exactly what it was he was looking at.

Not Pavel Chekov, not the golden bright-eyed genius sitting at the helm of the Enterprise at too-goddamned-young years old. But not a misshapen column of loose atoms. Not the formless pile of flesh and blood McCoy had feared.

Instead it was...well, like McCoy first thought. It was half of Chekov.

Short and small, eyes too wide in his face, cheeks too full. Hair a shade paler than it had been an hour ago, longer.

The black and gold of his uniform was sagging and hanging over his smaller shoulders. His eyes were clear green, too god-damned round. Scared. But alive.

This miniature Pavel looked around, his breath escaping faster and fear coming in to his features. He stepped back, and stumbled over the dragging leg of his uniform slacks.

When he spoke it was a high, clear voice. A child's voice.

_"Gde ja? Kto vy?_"

Considering what they had feared, this was disconcerting to say the least.

Chekov – or the kid version of Chekov's molecules, at least – didn't panic, which was admirable given where he had suddenly appeared and how many strange grown men were currently gaping at him like he was the second coming.

He just stared at them with baffled, scared eyes and spoke again, tremulous. _"Puzhalsta...ja zabludilsja. P-pomogite. Puzalsta...?_"

Jim ended up summing it up in that succinct way of his: "Huh."

Seconds ticked by, and the kid backed up another step and looked above him and around him, nervous but still not as scared as McCoy thought he maybe should have been.

And as they watched him the heavy atmosphere in the transporter room seemed to ease, if only a little. Spock studied the kid and the pad and the control panel with an ever-rising eyebrow, and Scotty was staring with an opened mouth that was starting to turn up at the corners.

Not a damned thing funny about it, though, and McCoy regarded the kid. It was obviously Chekov, and given the appearance and language this wasn't just an alteration of his body. His mind was altered as well. Regressed, somehow.

So, okay. He was the genius who saved McCoy's ass, but he was a scared kid who was suddenly in a strange place.

McCoy spoke hesitantly, eyes on the kid. "Spock, does Uhura speak Russian?"

"Yes, I believe she--"

"Kirk to Uhura. Get down to the transporter room. Now." Jim's hand fell from the communicator still strapped to his wrist.

The kid looked at them all with a shifting expression, and he stumbled forward then. "You speak Standard?"

McCoy blinked in surprise, but edged towards the step up to the pad and tried on a smile. "Yeah, we do. How much do you understand, kid?"

Chekov's too-young face crinkled in an instantly-familiar offended look. "I understand," he said simply. His accent was ridiculously thick – even more than the eighteen-year-old version – but he spoke evenly and without the hesitation of someone who was using a strange language.

Well, that was a relief. McCoy glanced back, but Jim and Scott were still gaping and Spock was staring at the kid like he was a particularly interesting slide under a microscope, so McCoy turned his back on them again.

"Okay. Good. That's good. Um..." Shit. How did they...what was the best...

There was no god damned protocol to handle half the shit that went down on Jim Kirk's starship, and it got really irritating sometimes.

The kid's wide eyes stayed on him, fear already fading into curiosity.

"What's your name, kid?" McCoy asked finally, figuring it was best to see just what of Chekov's brain was still in there.

"Pavel Andreivitch Chekov," came the instant response. The kid even seemed to straighten as he said it.

McCoy almost smiled despite himself. "How old are you?"

"I am seven. Almost. How old are _you_?" Chekov spoke again fast, though: like the older version of himself, he got right to the heart of the problem. "Where am I?"

McCoy started to answer, but what the hell answer was there, really? He glanced back, glowering at the useless mutes behind him. "Any time you want to step in here."

"Captain..." Spock spoke up then as if cued. "It would appear that the transport--"

Jim managed to shut his mouth, but his eyes stayed locked on the kid. "Spock, are you about to tell me something I can plainly see for myself?"

Spock hesitated. "That's likely."

"Okay. Maybe you should say it anyway."

"It appears the transporter has somehow altered Ensign Chekov's atomic makeup. Regressed him, somehow, to an earlier version of himself."

"Useful. Thanks." McCoy rolled his eyes. He looked back at the kid. "How do we undo it?" he asked quietly.

Chekov looked at him, and Spock, and back at him. His brow was furrowed too seriously, and McCoy could tell that he was listening to every word and absorbing it. No confused rambling or gasping out frightened questions. Not this kid. He would piece it together on his own if he had to.

Chekov would have been a genius at age six same as age eighteen.

The lack of answer to his question didn't escape McCoy's notice, and he straightened suddenly and looked down at the kid. "Okay, look, we're going to leave the chatterboxes here to work out some formulas or whatever. I think we need to get you to sickbay and get a look at you."

"Sickbay." Mini-Pavel regarded him, and looked around again in sudden new interest. "I am on a starship."

"Yeah, you are." McCoy reached out and clapped a hand on the kid's shoulder. He couldn't help himself – he could see his own little Jo in this kid. She was about seven when things went to hell and he lost her.

"Come on, kid. If I know you you're going to have a ton of questions and you're gonna want to see things for yourself."

Chekov moved awkwardly in his dragging uniform pants, but looked up at McCoy solemnly. "_Do _you know me?"

"Yep."

The doors slid open, and the relative calm was disturbed by two sets of footsteps as first Sulu and then Uhura came pounding into the room.

"Captain? What is going on down here?" Sulu looked around, his normally measured and calm face clouding. He scanned the room for Kirk, and though his eyes must have passed over Chekov he didn't seem to register the kid's presence. "Pavel goes tearing off the bridge without a word to anyone, and Scotty says we have to study the atmosphere or we're never going to..."

Uhura, unlike Sulu, had spotted Pavel and then stuck on him, her mouth dropping open in surprise.

Sulu was slower, but he caught up. As his words trailed off his gaze went back to the transporter pad, and he froze in astonishment.

McCoy sighed. "Okay, you guys hang around and play what-the-hell-just-happened, I'm getting the kid here up to sickbay to make sure his atoms are stable or whatever."

No one argued, but Sulu managed to get out a strangled, "Pavel?" as they passed, which made the kid stumble and look back at him.

But he kept moving when McCoy kept steering him, and when the door opened and closed behind them he looked around with an instant, intent curiosity that must have overwhelmed any questions he may have had.


End file.
